The present invention relates to a circuit board and, more particularly, to a circuit board prepared using a resin as a base material.
Thermosetting resins are conventionally used as base materials to prepare cirucuit boards such as paper-phenol circuit boards, glass mat-polyester circuit boards and glass-epoxy circuit boards. In these circuit boards, a copper-clad thermosetting laminate is used, a resist pattern is printed, and unnecessary copper portions are selectively dissolved and removed by etching, thereby preparing a circuit pattern.
In order to obtain a circuit of high packing density, a multi-layer circuit pattern is produced. In this case, in order to connect circuit patterns between every two adjacent layers, nonelectrolytic plating techniques are frequently used. Etched copper-clad circuit boards are alternatively bonded by thermocompression with prepregs. Holes are formed in the multi-layer laminate and the wall surfaces of these holes are then subjected to nonelectrolytic plating. The upper and lower surfaces of the multi-layer laminate are then etched to obtain predetermined patterns. In this manner, the interlayer circuit patterns are electrically connected. According to this conventional technique, the process is complicated, and the circuit board is of high cost.
Another conventional multi-layer circuit board is a thick film circuit board obtained such that a conductor paste is printed and sintered on an insulating substrate, and an insulating paste is printed and sintered on the conductor pattern. The above process is repeated to prepare this multi-layer circuit board. According to this process, the through hole process, i.e., coating of the conductor on the wall surfaces of the holes, need not be performed, thus resulting in a simple process. However, when printing is repeated to increase the thickness of a resultant laminate, steps are accumulated. In practice, printing cannot be performed for a laminate having four or more layers.
In all the conventional multi-layer circuit boards, the insulating substrate surface does not have the same level as that of the circuit pattern surface. The circuit pattern surface often extends above the insulating substrate by several tens of microns. Therefore, in a electric component subjected to sliding along the circuit board (e.g., coupling operation of a connector, or sliding of contacts of a switch and a variable resistor), the operation becomes unstable, and mechanical wear and electric are occur.